1. Field of the Invention
The present invention relates to a process for the catalytic oxidation of liquid cycloparaffins and, in particular, to an improvement in the catalytic oxidation of liquid cycloparaffins, especially cyclohexane, in which the oxidation catalyst is a heavy metal compound in combination with an N-heterocyclic compound.
2. Description of the Prior Art
The oxidation of cycloparaffins to produce useful partial oxidation products, for example, the oxidation of cyclohexane to cyclohexanol and cyclohexanone, is known as one important step in the manufacture of nylon intermediates, for example, adipic acid. In the case of adipic acid manufacture, it has been found to be preferable to oxidize cyclohexane to adipic acid in a two-step oxidation process, i.e., to oxidize cyclohexane to a mixture of cyclohexanol and cyclohexanone and to thereafter oxidize that mixture to adipic acid by means of a nitric acid oxidation process.
A process in which cyclohexane is oxidized in the liquid phase to cyclohexanol and cyclohexanone at low conversion and high yields was disclosed by Donald J. Loder in Canadian Pat. No. 401,788, which issued Dec. 30, 1941. In the Loder process, the preferred catalysts include cobalt alkanoate, especially cobalt naphthenate. The yields of cyclohexanol and cyclohexanone obtained by the Loder process were considerably higher than had been achieved in earlier processes not employing catalysts or initiators. While Loder's process when operated on a noncommercial scale can be made to give yields of cyclohexanol plus cyclohexanone from cyclohexane of 85-95%, in practical commercial operations it is often necessary to compromise on yield in favor of other process parameters in order to have a continuous process which can be run in an economical manner.
It is known that the quantities of useful oxidation products and the by-products of the oxidation of cycloparaffins depend on, in particular, the temperature, residence time, oxygen concentration and flow rates of cycloparaffin, and the like. An improvement in the method for the oxidation of cyclohexane and for the control of the oxidation products so obtained is discussed by K. Pugi in Canadian Pat. No. 745,204, which issued Oct. 25, 1966.
A process for the catalytic oxidation of cycloparaffins to the corresponding cycloalkanol and cycloalkanone in the presence of a catalyst that is, for example, a cobalt monoalkylphosphate and/or cobalt dialkylphosphate, is disclosed by A. Kuessner et al. in U.S. Pat. No. 3,917,708, which issued Nov. 4, 1975.
Commercial processes for the oxidation of liquid cycloparaffins are usually operated on such a large scale that there are significant incentives to increase the efficiency of the process, especially with respect to the unit cost, e.g., the cost per ton of product, for the manufacture of useful oxidation products. The percentage yield of useful products, per mole of cycloparaffin oxidized, and the amount of cycloparaffin oxidized, i.e., the conversion, obtained as a result of one pass of cycloparaffin through the oxidation zone of the process, are both important process parameters. Such parameters have a major effect on the productivity of the process, i.e., the amount of useful oxidized products formed in the process in a given period of time, which is a prime factor in determining the unit cost for the manufacture of useful oxidation products.
It has now been found that cycloparaffins may be catalytically oxidized to useful partial oxidation products by using a process in which the oxidation catalyst is a heavy metal compound in combination with an N-heterocyclic compound.